This invention relates generally to silylated polyurethane polymers, and more particularly to silane-terminated polyurethane polymers with improved mechanical properties, to silane- and low molecular weight terminator-terminated polyurethane polymers, to sealant compositions containing such polymers, and to methods of making them.
Generally, conventional urethane-based adhesives and sealants have better physical properties, including tensile strength, elongation, and tear strength, than do adhesives and sealants made with silylated polyurethane capped with common capping agents such as aminotrimethoxysilanes and isocyanatotrimethoxysilanes. This makes it difficult to use silylated polyurethane-based adhesives and sealants in some applications, despite other advantages that they have over urethane-based products.
There are currently two approaches used to improve the mechanical properties of silylated polyurethanes. One solution has been to cap the polyurethane completely using difunctional silanes as capping agents. However, the cure rate of adhesives and sealants made with this type of silylated polyurethane decreases unacceptably in some cases. The other approach is to cap the polyurethane completely using certain special capping agents which reduce the cross-linking density. These include silanes having the general formulation 
such as N-phenyl-gamma-aminopropyltrimethoxysilane. These special silanes are costly to use because of their high prices and high molecular weights, and the improvement in properties they provide is limited.
U.S. Pat. No. 3,627,722 (Seiter) discloses a partially capped polyurethane sealant composition. Seiter states that between 5 and 50% of the NCO groups should be capped. According to Seiter, capping more than 50% of the NCO groups produces a soft polymer with less cohesive strength than when less than 50% are capped.
Therefore, there is a need for economical silylated polyurethanes with improved mechanical properties for use in adhesives and sealants.
This need is met by the present invention which provides a novel silylated polyurethane polymer having improved mechanical properties which can be used in sealant compositions, and a method of making the same.
The silylated polyurethane polymer includes a polyurethane pre-polymer having between greater than about 50 to about 95% of the NCO groups terminated with silane groups. The polymer has improved mechanical properties, including tensile strength, elongation, and tear strength. The polymer can also have between about 5% to less than about 50% of the NCO groups terminated with low molecular weight terminators. Low molecular weight terminators include an active hydrogen which can react with the NCO groups.
The silane groups are preferably selected from 
where R represents a divalent organic moiety, R1 represents a C1-6 alkyl group, and R2 represents hydrogen, a C1-6 organic moiety, or a second RSi(OR1)3 which may be the same as or different from the first such group.
The low molecular weight terminators have groups preferably selected from functional end groups except NCO groups, and inert end groups. The inert end groups are preferably selected from 
where R3 represents an organic moiety whose molecular weight is less than 1000, and R4 represents hydrogen or R3. The molecular weight of R3 is more preferably less than 100, and R3 is most preferably a methyl, ethyl, or propyl group.
The functional end groups are preferably selected from 
where R4 represents hydrogen or an organic moiety whose molecular weight is less than 1000, R5 represents a divalent organic moiety, and Z represents any functional group except NCO groups. The molecular weight of R4 is more preferably less than 100, and most preferably R4 is a methyl, ethyl, or propyl group.
Z is preferably selected from 
where R4 represents hydrogen or an organic moiety whose molecular weight is less than 1000, and more preferably less than 100. R4 is most preferably a methyl, ethyl, or propyl group.
Another aspect of the invention are sealant and adhesive compositions including the silylated polyurethane polymer as described above. The sealant and adhesive compositions will typically include additional compounds, such as fillers, reinforcing agents, solvents, catalysts, rheology modifiers, ultraviolet light screening compounds, and adhesion promoters.
Another aspect of the invention is a method of making the silylated polyurethane polymer described above. The method includes reacting a polyurethane prepolymer having NCO groups with silane capping agents so that between greater than about 50 to about 95% of the NCO groups are terminated with silane groups. The method optionally includes reacting the polyurethane prepolymer with a low molecular weight terminator so that between about 5 to less than about 50% of the NCO groups are terminated with low molecular weight terminators.
Preparation of isocyanate terminated urethane prepolymers, and silylated polyurethane with silane capping agents, are well known in the art. See, e.g., U.S. Pat. Nos. 3,627,722, 3,632,557, 3,979,344, and 4,222,925, which are incorporated herein by reference. Curing of the polymer results from hydrolysis of the silane end groups and condensation of Sixe2x80x94OH moieties.
In the present invention, a majority of NCO groups are capped with common silane capping agents. The remaining NCO groups are either left unreacted, or are capped with low molecular weight terminators. The low molecular weight terminators can form inert end groups or introduce different functional groups into the silylated polyurethane.
Between greater than about 50 to 95%, preferably between about 80 to about 95%, of the NCO groups can be capped with common suitable silane capping agents. There can be between about 5 to less than about 50%, preferably about 5 to about 20% uncapped NCO groups. The percentage of capped NCO groups depends on the chemical structure of the isocyanate terminated urethane prepolymer. That is, it depends on factors including the type of raw materials, such as toluene diisocyanate (xe2x80x9cTDIxe2x80x9d) or diphenyl-methane diisocyanate (xe2x80x9cMDIxe2x80x9d) and polyester polyols or polyether polyols, functionality, molecular weights, and also on the desired
Suitable silanes include, but are not limited to, silanes having the following general formulas: 
where R represents a divalent organic moiety, R1 represents a C1-6 alkyl group, and R2 represents hydrogen, a C1-6 organic moiety, or a second RSi(OR1)3 which may be the same as or different from the first such group.
The portion of the NCO groups which are not capped with the silane may optionally be capped with low molecular weight terminators. Generally, the low molecular weight terminators can be any substance containing hydrogen which will react with an NCO group. The low molecular weight terminators can form inert end groups or introduce desirable functional groups onto the polymers. Low molecular weight terminators which form inert end groups include, but are not limited to, the following: 
where R3 represents an organic moiety whose molecular weight is less than 1000, preferably less than 100, and most preferably R3 is a methyl, ethyl, or propyl group, and R4 represents hydrogen or R3. Low molecular weight terminators which can be used to introduce different functional groups include, but are not limited to, 
where R4 represents hydrogen or an organic moiety whose molecular weight is less than 1000, R5 represents a divalent organic moiety, and Z represents any functional group except NCO groups. The functional groups include, but are not limited to, 
where R4 represents hydrogen or an organic moiety whose molecular weight is less than 1000.
The silane capping agents and the low molecular weight terminators can be added together or separately. The sequence in which the reactants are added depends on their relative reactivity.
Adhesives and sealants made using the polyurethane prepolymers of the present invention will typically include fillers such as carbon black, and catalysts such as dibutyltin diacetate. Other fillers and catalysts can also be used. The adhesives and sealants can also contain additional components including adhesion promoters, rheology modifiers, and ultraviolet light screening compounds, and other components if desired.